Vehicles using lean burn engines such as diesel engines include, in their exhaust gas passages, filters that collect particulates (particulate matters, i.e., PMs, such as carbonaceous particulates) in exhaust gas. When a large amount of PMs are accumulated on a filter, the filter is clogged. To prevent the clogging, the accumulated PM amount is estimated based on, for example, the pressure difference between pressure sensors provided to sandwich the filter. When the accumulated PM amount reaches a predetermined value, fuel injection control (e.g., increase in the amount of fuel or post injection) of the engine causes unburned HC to be included in an exhaust gas and to be supplied to an oxidation catalyst. This oxidation catalyst is intended to burn unburned HC so as to increase the temperature of the exhaust gas that has reached the filter for removal of PMs through combustion. To promote the PM combustion, a catalytic material is generally loaded on an exhaust gas passage of the filter.
For example, Patent Document 1 describes that a catalyst layer including a composite oxide of Zr and a rare earth metal except Ce and a composite oxide of Ce and a rare earth metal or an alkali earth metal is provided on an exhaust gas passage wall of a filter.
Patent Document 2 proposes that a catalyst layer including a precious metal-doped CePr-based composite oxide and a composite oxide of Zr and a rare earth metal is provided on a passage wall of a filter.
Patent Document 3 shows a filter in which a catalyst layer including a Zr-based composite oxide containing Zr, Nd, and a rare earth metal except Nd and Ce, e.g., La or Pr, is provided on an exhaust gas passage wall. As described above, the catalyst layer including a mixture of a Zr-based composite oxide and a Ce-based composite oxide increases the rate of PM combustion and reduces the time for reusing the filter, thereby enhancing fuel economy.